1. Field of the Invention
The present invention relates to an optical coherence tomography observation apparatus, a method for determining a relative position of images, and a program for determining the relative position of images.
2. Description of Related Art
Generally, in cell culture of the related art, cells which are two-dimensionally grown are observed, and the properties are evaluated. However, in order to express essential properties of the cells, the importance of growing the cells three-dimensionally has been becoming clear in recent years.
With the above-described background, expectations with respect to a method which observes three-dimensional structures of the cells have increased.
Optical Coherence tomography (OCT) which uses low coherence light has advantages such as a resolution on the order of μm in a depth direction and an observation range on the order of mm in the depth direction, and is a promising technology which observes three-dimensional structures of cells (Non-Patent Document 1).
Research and development of OCT has advanced since the 1990s, and OCT is generally divided into two kinds of OCTs such as a time domain method and Fourier domain method.
In the OCT of the time domain method, a light beam is divided into signal light and reference light, the signal light scattered from cells and the reference light reflected from a reference mirror become interfere with each other, and interference light is generated. At this time, since coherency of the light beam used in a light source is low, only the signal light from a specific depth which is scattered from the cells becomes interfere with the reference light. Accordingly, the depths of the cells exhibiting the interference can be changed by changing the light path length of the reference light, and three-dimensional structures of the cells can be observed.
However, in the OCT of the time domain method, since information in the depth direction is obtained by moving the reference mirror in an optical axis direction, the measurement time is long, and thus, currently, the OCT of the time domain method has not been adopted. On the other hand, in the OCT of the Fourier domain method, since the information in the depth direction can be collectively obtained, the measurement time is short, and thus, currently, the OCT of the Fourier domain method has become a mainstream method (Non-Patent Document 2).
Here, the OCT of a spectrum domain method (SD-OCT) which is a kind of OCT of the Fourier domain method is described as an example. In the SD-OCT, the observation range in the depth direction is limited by the spectral resolution of a spectroscope and the focal depth of an objective lens.
For example, it is assumed that the observation range in the depth direction determined by the spectral resolution of the spectroscope is the same as the observation range in the depth direction determined by the focal depth of the objective lens. When the observation range in the depth direction is widened by increasing the spectrum resolution, the focal depth of the objective lens also needs to be deep. In order to make the focal depth of the objective lens deep, the numerical aperture (NA) of the objective lens needs to be decreased. However, since lateral resolution (the resolution in a horizontal direction) is inversely proportional to the NA of the objective lens, the lateral resolution (the resolution in the horizontal direction) necessarily decreases if the NA is decreased.
Conversely, when the observation range in the depth direction is narrowed by the decreasing of the spectrum resolution, the observation can be performed with high lateral resolution using an objective lens having a shallow focal depth and a large NA. That is, the observation range in the depth direction and the lateral resolution are in a trade-off relationship.
From the above-described relationships, in order to widen the observation range in the depth direction in a state where the lateral resolution is maintained, generally, a method is adopted in which tomographic images (hereinafter, referred to as “OCT images”) having narrow observation ranges in the depth direction are obtained in plural sheets and the plurality of sheets of the tomographic images are connected to each other Non-Patent Document 3).